Synology Mail-Server "User unknown in local recipient table"

Question

I have set up a Synology DiskStation as a SOHO email server for my small business.

$uname -a 
Linux servername 3.10.102 #15132 SMP Thu Jun 29 07:35:19 CST 2017 armv7l GNU/Linux synology_armada38x_ds216j

The "Mail Server" MTA app seems to be based on Postfix. The "Mail Station" webmail app seems to be based on Roundcube.

The latter is what is configured to collect mail from other POP3 servers.

Configuration and Administration is through Synology's web-interface.

I have set up two users, each can send and receive mail for [e-mail] using either the Webmail service from a web-browser on a PC or by using the IMAP service from Thunderbird on a PC.

The Diskstation sucessfully collects inbound mail from an ISP's POP3 service and sucessfully delivers outbound mail to the ISP's SMTP service.

The users have their full email-address listed in "Users" in the Control panel.

Name      Email                    Description    Status
John      [e-mail]    John Doe       Normal
Jane      [e-mail]  Jane Smith     Normal

However when one user sends mail to the other internally, it works OK if they send to [e-mail] but Thunderbird reports "User unknown in local recipient table." if they send to [e-mail].

An error occurred while sending mail. The mail server responded:  
5.1.1 <[e-mail]>: Recipient address rejected: User unknown in local recipient table.
 Please check the message recipient "[e-mail]" and try again.

How can I resolve this problem?

Answer 1

By a process of trial and error I worked out that, from the administration web-page, I needed to go into the "Mail Server" App, select "Aliases" and create an alias for each user.

  Alias name:      John.Doe
  External mail box:  [e-mail]
  User
      [ ] admin
      [ ] jane
      [ ] guest
      [/] john

This seems to have created/populated a file /volume1/@appstore/MailServer/etc/aliases and /volume1/@appstore/MailServer/etc/aliases.db.

The aliases file contains

MAILER-DAEMON: postmaster
postmaster: root
John.Doe: [e-mail], john
Jane.Smith: [e-mail], jane
root: admin

The role of the "External mail box" value isn't clear, but things seem to be working OK now. I will update this answer if any adverse consequences become visible.